1. Field of the Invention
This invention relates generally to blown film extrusion lines, and specifically to signal filtering for improved cooling systems for use with blown film systems.
2. Description of the Prior Art
Blown film extrusion lines are used to manufacture plastic bags and plastic sheets. A molten tube of plastic is extruded from an annular die, and then stretched and expanded to a larger diameter and a reduced radial thickness by the action of overhead nip rollers and internal air pressure. Typically, air is entrained by one or more blowers to provide a cooling medium which absorbs heat from the molten material and speeds up the change in state from a molten material back to a solid material. Additionally, blowers are used to provide air pressure which is utilized to control the size and thickness of the film tube. One type of blown film extrusion line utilizes an air flow on the exterior surface of the film tube in order to absorb heat. A different, and more modern, type of blown film extrusion line utilizes both an external flow of cooling air and an internal flow of cooling air in order to cool and size the film tube.
Whether the blown film tube is cooled from either the interior surface, the exterior surface, or both, one common problem in blown film extrusion lines is that of obtaining precise control over the diameter of the extruded film tube. Tight control over the diameter insures uniform product dimensions, which includes the size of the extruded product, as well as the thickness of the plastic material. Acoustic sensors may be utilized to gauge the diameter of the product without deforming the product, in the manner of mechanical dimension sensors. One drawback with utilization of ultrasonic sensors is that they are very sensitive to ambient noise, as well as flutter or slight vibration of the extruded film tube.